In Phase Contrast Magnetic Resonance Imaging (PC-MRI), background phase offsets occur due to eddy currents and concomitant gradient terms, which can introduce substantial errors in the velocity measurements. A number of steps can be taken to minimize these errors, but background phase errors still remain and the accuracy of the PC-MRI techniques often fails to meet clinical needs for integrated flow measurements. Two solutions to compensate for residual eddy currents are to perform a separate phantom scan with the same sequence, imaging parameters, orientation, and scan location, or to physically measure the magnetic field during the scan. Both of these methods require additional time on the scanner, and do not compensate for errors specific to the anatomy being imaged. Furthermore, eddy current induced phase offsets may not be sufficiently stable over time for correction methods using separate measurements to work effectively.
Phase correction can alternatively be performed as a post-processing step. The most common post processing correction technique involves estimating the phase variation in stationary tissue and subtracting a fitted surface, typically polynomials or basis functions. Previous work shows that a high order polynomial may be beneficial. An alternative technique, used in susceptibility weighted imaging, is to assume that the phase inhomogeneities are low frequencies in k-space and can be filtered out using a high-pass k-space filter. In practice, this can be implemented by performing a complex division of the original image and a low-pass filtered version of the original image. This approach has been applied previously for PC-MRI, but only using the spatial dimensions.